The importance of canceling acoustic echo in audio signals generated by a full-duplex conferencing system is well known to audio communications engineers. Generally, acoustic echo results when a microphone and loudspeaker that are linked to the same conferencing system are proximate to each other. Loudspeaker output is received by a microphone and transmitted back to an individual speaking at a remote location who then hears their words a second time after the round trip delay. This acoustic echo can be very disruptive during the course of a conversation, as speakers at both a local and remote location may have to wait for the acoustic echo to subside before speaking again. To solve this problem, audio communication engineers have developed a variety of acoustic echo cancellation methods that have the effect of removing the acoustic echo component from the signal received by a microphone from a loudspeaker. Generally, acoustic echo cancellation (AEC) operates as follows: audio signals received from a remote location are framed to form a reference signal, this audio signal is then played by a loudspeaker, a microphone receives the acoustic echo from the loudspeaker and the resulting microphone signal is framed. The earlier generated reference signal is compared to the microphone signal and the two signals are summed at 180° out of phase and the resulting signal is transmitted back to the other end without any echo. Typically, it is very difficult to completely eliminate acoustic echo in an audio signal and so some back end processing to suppress the gain of the remaining echo in the return audio signal can be applied. As such, acoustic echo cancellation typically is composed of two separate processing steps. A first step linear echo cancellation step applies filters to filter out as much of the echo component in the signal as possible. But typically, echo cancellers are not able to completely eliminate the echo and so a second step applies the audio signal to some non-linear processing which “suppresses” or decreases the gain of the signal thereby suppressing the remaining remnants of the echo before sending an audio signal to the remote location.
A monophonic audio conferencing system suitable for use in a conference room typically employs one or more microphones and a single loudspeaker system to play audio received from a remote conferencing system. However; for larger rooms designed to accommodate many individuals each with their own microphone, it is usually advisable to employ a public address speaker system that both plays remote audio and amplifies or reinforces the local audio signals. With such an arrangement, all of the participants in the audio conference can be heard as they speak into their microphone by all of the other participants in the same room. One problem that arises when local sound reinforcement is employed in a conferencing system is that the reinforced audio is received by the microphones, creating a feedback loop and the potential for “howling”. This problem can be resolved by simply turning the sound reinforcement gain down until the howling stops. Unfortunately, turning the sound reinforcement gain down can defeat the purpose of reinforcing the sound, resulting in the locally reinforced sound being to low to be easily heard by all of the participants in the room. Automatic feedback eliminator (AFB) methods have been employed to resolve this problem. Another problem associated with reinforcing local audio is that an audio conferencing system typically does not discriminate between local and remote audio signals. As the conferencing system is not able to discriminate between local or remote audio, it will run both types of audio signals through the same acoustic echo cancellation process. This is fine for the audio signals generated at a remote location, but AEC processing denigrates the quality of the local audio signal that is transmitted to a remote location and this processing also limits the local sound reinforcement gain that can be realized in the conferencing system.
Prior art solutions to this local sound reinforcement feedback problem have focused on utilizing notch filters with a singing detector that tracks and filters out the set of singing frequencies that cause the acoustic feedback problem. This approach can improve the gain of a local sound reinforcement system by as much as 6 dB. However, this solution typically injects comb filtering artifacts into the local audio signal which can be perceived as hollow and suppressed audio. Another solution to this problem is to employ an acoustic echo canceller to remove the feedback from the local audio signal. Unfortunately, this approach creates another problem; namely, acoustic echo cancellation attempts to eliminate the local audio signal which has the audible effect of clipping the local audio signal which compromises both the reproduction of the local audio at the far end of a conference and the reproduction of the reinforced local audio at the near end. Nothing in the prior art teaches how to process a local audio signal, in the presence of sound reinforcement, such that the quality of the local audio signal is maintained and the gain of the sound reinforcement system is maximized.